Tuesday, March 20, 2012

A Floral Attraction

One of the first large insects to be conspicuous at this time of year is the family of hoverflies known as Drone Flies (Eristalis spp.). The availability of pollen and nectar from the newly emerged catkins of willow provide a bountiful supply of food for the flies, and as the number of plants in flower increases, so will their numbers. Their common name, Drone Flies, comes from their resemblance to bees, a form of Batesian mimicry that no doubt gives them some degree of predators. Their larvae employ no such disguise and are quite distinctive, possessing a long respiratory tube attached to their anal gills that enables respiration as they are buried in the mud and earning them the name 'Rat-Tailed Maggots' (1).

Eristalis tenax covered in Willow pollen after feeding

Yet, watching one of the most common of the Drone Flies currently in flight, Eristalis tenax, flit from flower to flower in search of food does beg the question how do they know where the food is. How do they know that a flower is a flower? The answer is a variety of olfactory and visual signalling structures used by plants to attract insects. E. tenax in particular is attracted to yellow flowers, or yellow spots on flowers, with light in the colour range of 510-600 nm (green/yellow light) releasing the extension of the proboscis (2). Flowers use guide lines to attract the flies from a distance, which are also most effective when yellow (3). Attraction to the yellow colour is due to the fact that this is the colour of pollen, food for E. tenax. This explains it eliciting a feeding reaction.

Identification of Eristalis spp. is notoriously difficult, so I was very grateful to Stuart Dunlop at the Donegal Wildlife blog for his tips on identifying E. tenax.

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